A tire has a structure formed by stacked layers of various materials, such as rubber, chemical fibers, and steel cords. If this layered structure has a non-uniform portion and the tire is filled with air, a raised portion called “bulge”, or a recessed portion called “dent” or “depression”, is formed in an area where resistance to pressure is relatively weak. In inspection, a tire with a shape defect, such as a bulge or dent, needs to be excluded from shipment due to safety concerns or problems with appearance.
Therefore, in the final stage of tire production (i.e., in an inspection step after tire vulcanization), tire surfaces, particularly sidewall surfaces, are inspected for defective unevenness or shape defects. The sidewall surfaces of a tire have indication marks (normal uneven marks) that indicate the model and size of the product, the logo of the manufacturer, etc. Therefore, in the process of inspecting the sidewall surfaces for shape defects, it is necessary that such indication marks be not erroneously detected as shape defects.
Conventionally, inspections for such defective unevenness or shape defects would be manually performed, both visually and by touching. In recent years, efforts have been underway to develop automated techniques, such as laser distance sensors, three-dimensional shape measuring apparatuses, and camera-based image inspections, and inspection techniques which are not affected by the presence of normal uneven marks.
For example, Patent Literature (PTL) 1 discloses a tire shape detecting apparatus that detects a surface shape of a tire by picking up an image of line light projected onto the surface of the tire rotating relatively, and performing shape detection by a light section method on the basis of the picked-up image. The tire shape detecting apparatus includes line light irradiation means for continuously projecting a plurality line light beams from directions different from a detection height direction in one light section line such that the one light section line is formed on the surface of the tire; and image pickup means for picking up images in a direction in which chief rays of the plurality of line light beams projected onto the surface of the tire are specularly reflected from the surface of the tire.
In particular, this tire shape detecting apparatus is configured to continuously project a plurality line light beams onto the tire surface, pick up images of the plurality of projected line light beams, and detect a tire surface shape.
PTL 2 discloses a method for inspecting three-dimensional shapes of one or more uneven (embossed) marks in a tire surface. The method includes a step of measuring heights of unevenness for each of area elements, including these marks, in a predetermined tire surface region to acquire unevenness distribution data; a step of identifying a tire surface portion corresponding to a mark model in the tire surface region, the mark model being prepared in advance as a mark template for each of the marks, from three-dimensional shape data of the mark model and the acquired unevenness distribution data; and a step of determining, for each of the marks, a degree of coincidence between the unevenness distribution data of the identified tire surface portion and the three-dimensional data of the mark model, and determining whether to accept the three-dimensional shape of the mark on the basis of the degree of coincidence.
In particular, this method for inspecting tire uneven marks performs inspection for detects by calculating a degree of coincidence between three-dimensional unevenness distribution data acquired by irradiating a tire surface with sheet light and three-dimensional shape data of a mark model generated from CAD data. In this technique, which determines whether to accept a normal uneven mark (e.g., text), a mark model prepared in advance as a template for the normal uneven mark is used as teaching data. The template is generated from tire CAD data or mold CAD data.